Tracking how engineering materials deform to extend their lifetime

19-01-2021

Scientists led by the Western University in Canada have discovered important clues about the deformation process in engineering materials used in cars or nuclear reactors. Their findings are leading to new models to predict lifetime of these materials.

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The automobile industry, in its quest for light materials to reduce the carbon footprint, uses magnesium alloys because of their weight in many components such as gearboxes, driver’s air bag housings as well as in steering wheels, seat frames and fuel tank covers, for example. In the nuclear domain, zirconium alloys are used to build reactors. Estimating the lifetime of these materials before they fail is crucial, but for that scientists need to characterise key parameters that control their mechanical behaviour.

Whilst being used, these alloys carry thermomechanical loads, which may cause both recoverable, also known as elastic (when material goes back to its original form), and irrecoverable, also known as plastic (when material does not go back to its original form) deformation. Plastic deformation allows materials to be formed, for example, rolled into a sheet, but it can also ultimately lead to failure of a component.

One of the processes that plays a role in the lifetime of a material is called twinning, which is a reorientation of a portion of a crystal. It is and important process for the properties of hexagonal close-packed metals and alloys, such as magnesium and zirconium, because it can accommodate some additional plastic deformation in the material. Twins are formed when load is applied onto a material and can sometimes disappear when the load is removed or reversed.

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A crystal plasticity model created by the scientists, where colours represent grains that the team reconstructed based on the experimental data we measured at ESRF. Credit: H.Abdolvand.

Twins can be good and bad. They can improve ductility, because without them the material would break, but sometimes, depending on the loading conditions of the material, and microscale stress field around them, they can cause fracture.

“The role of twins is extremely important, and at the moment it is challenging for the scientific community to predict them because their onset is a rapid process. With my team and researchers at the ESRF and Arts et Metiers ParisTech we wanted to find out how they initiate, under which conditions and what they do to the materials’ properties”, explains Hamidreza Abdolvand, scientist at the Western University in Canada and corresponding author of the paper. 

The team came to the ESRF’s ID11 beamline and used the technique of 3D synchrotron X-ray diffraction. “The goal was to understand what happens at the atomic level and to relate that to how the material works”, explains Jon Wright, scientist in charge of beamline ID11. The experiment itself was quite challenging: “We had very thin samples and needed to apply load onto them, so we could see very small twins very early on at the onset of plasticity”, explains Marta Majkut, scientist at the ESRF and part of the team.

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Scientists Jon Wright (left) and Marta Majkut on the experimental hutch of ID11. Credits: D. Chenevier

After the work done at the ESRF, the team combined the results with crystal plasticity modelling to study formation and annihilation of twins. “We found that at the early stages of plasticity, thin twins are very stressed, yet they relax with further loading”, adds Abdolvand.

These results complement previous findings published in 2007 and 2008, where scientists claimed that thick twins were relaxed.

All this outcome is important to develop new models that allow manufacturers and researchers to predict what will happen to a material and at what point this material will start failing or fracturing. “We have submitted a new model, under review at the moment, to be used for materials that twin”, concludes Abdolvand.

Reference:

Louca, K., et al,Commun Mater 2, 9 (2021). https://doi.org/10.1038/s43246-020-00105-y

Text by Montserrat Capellas Espuny

Top image: Many car components, such as the steering wheel, the gearbox casing, or the seat frames are made of magnesium alloys.